|Quoting EBJ1248650 (Reply 2):|
The first two digits of F-15C/D serial numbers are 78 and I understand the last ones had 86 as the first two digits.
tail numbers always have the US Government FY of the purchase order placed, under the exsisting contract. For example the 78 through 86 you referred to were for FY 1978 to FY 1986. That means the airplane's whos tail number starting with the 78 was ordered in FY 1978.
It has nothing to do with the aircraft model. FY 1977 would have been a difficult year to pick out a perticular model of the F-15, as F-15A/Bs were ordered in the first half of FY-77, and F-15C/Ds were ordered in the second half of FY-77. The airplane block numbers and tail number groupings were different.